Luminescent material and light emitting diode using the same

ABSTRACT

UV-blue excitable luminescent material consisting of a Eu-doped oxynitride host lattice with general composition MaI 2-x Si x O 4-x N x , wherein M is at least one of an alkaline earth metal chosen from the group Ca, Sr, Ba.

TECHNICAL FIELD

This invention relates to a luminescent material which is excitable inthe UV-blue part of the spectral region, and more particularly, but notexclusively to a phosphor for light sources, preferably for LightEmitting Diodes (LED). The phosphor belongs to the class of rare-earthactivated silicon oxynitrides.

BACKGROUND ART

So far white LEDs were realized by combining a blue-emitting diode witha yellow emitting phosphor. Such a combination has a poor colorrendition, which, however, can be improved significantly by using ared-green-blue system (RGB). Such a system uses for example a red andblue emitter in combination with a green-emitting aluminate phosphor,like SrAl₂O₄:Eu or BaAl₂O₄:Eu, with the possible addition of Mn to Eu,whose emission maximum is around 520 nm, see U.S. Pat. No. 6,278,135.However, the position of the excitation and emission bands of thesesaluminates is not optimum. They have to be excited by short UV in therange of 330 to 400 nm.

New interesting luminescent materials are α-sialon materials doped withEu²⁺. They have the structureM_(t)Si_(12-(m+n))Al_((m+n))O_(n)N_((16-n)) with M is Ca or Y or rareearth metal. The value t is given as m/val+, wherein val+ is the chargeof the valence of the ion M. For example, the val+ value of M=Sr²⁺ is 2.Thus the charge of the whole structure is fully compensated. For furtherdetails see for example “Luminescence properties of Tb, Ce, or Eu-Dopedα-Sialon Materials” by J. van Krevel et al., J. Sol. St. Chem., April2002, p. 19-24, and also “Preparation and Luminescence Spectra of Ca andrare Earth co-doped α-SiAlON Ceramics” by R.-J. Xie et al. in J. A.Ceram. Soc. 2002, p. 1229-1234, May 2002, and further on U.S.-Pub2002/0043926, which references all deal with Ca-Sialon of the α-Sialontype. Other types of sialon were not known at this time.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to provide a new luminescentmaterial in accordance with the preamble of claim 1. A further object isto provide a phosphor with a fine-tuned emission which can beefficiently excited by UV/blue radiation. A further object is to providea phosphor for use in an illumination device with at least one LED aslight source, the LED emitting primary radiation in the range from 360to 470 nm, this radiation being partially or completely converted intolonger-wavelength radiation by phosphors which are exposed to theprimary radiation from the LED. A further object is to provide anillumination device which emits white light and in particular has a highcolor rendering. A further object is to provide a high-efficiencyillumination device like a LED device which absorbs well in the rangefrom 360 to 470 nm and is easy to produce.

These objects are achieved by the characterizing features of claim 1 and10, respectively. Particularly advantageous configurations are given inthe dependent claims.

The conversion is achieved at least with the aid of an oxynitridephosphor which originates from the class of the Eu-activated or Eu,Mn-co activated aluminates. In more detail, the disadvantages of theprior art are overcome by incorporation of nitrogen in MAl₂O₄:Eu (M=Ca,Sr, Ba), resulting in an oxynitride phosphor, usually and preferablymaintaining the tridymite structure. However other structures are notexcluded. More specifically, (AlO)⁺ is partially replaced by (SiN)⁺giving the general composition: MAl_(2-x)Si_(x)O_(4-x)N_(x):(Eu orEu,Mn). The value x is set as x≧0,002 and at most x=1,5. In contrast tothe afore mentioned prior-art α-sialon type structures, which arederived from the nitride Si₃N₄, the structure here is more oxide-like,derived from a SiO₂ modification, and more specifically preferablyderived from a tridymite structure.

In contrast, α-sialon materials show a so-called nitridic structure, inother words they are based on Si₃N₄ wherein Si—N is partially replacedby Al—O and/or by Al—N. These different structures result in a differentluminescent behavior since possible activator sites in alpha-sialons andtridymites strongly differ in the local interaction with the surroundingions.

In case of Ca as main component of M a preferred value is 0,01≦x≦0,1. Incase of Ba or Sr as main component of M a preferred value is 0,1≦x≦0,7.The incorporation of nitrogen increases the degree of covalent bondingand ligand-field splitting. As a consequence this leads to a shift ofexcitation and emission bands to longer wavelengths compared to oxidelattices. The obtained phosphors show high chemical and thermalstability. More extended fine tuning of all relevant properties can beobtained by use of a cation M which is achieved by combining several ofsaid M metals (especially Sr and Ba), by further inclusion of Zn as partof cation M (preferably 10-40 mol-%), and/or at least partialreplacement of Si by Ge (preferably 5-25 mol %) and/or Al by Ga(preferably 5-25 mol %). Preferably, the metal M is mainly Ba and/or Srfor a green-emitting material, and mainly Ca for a blue-emittingmaterial. The amount of Eu doped to cation M is between 0,1 and 25%,preferably between 2 and 15% of M. In addition further doping with Mnfor fine-tuning of relevant properties is possible with an preferredamount of at most 50% of the Eu doping.

Since these materials can convert UV-blue radiation into blue-greenlight due to low-energy excitation bands, they can be applied forexample in white light sources (e.g. lamps), especially sources based onprimarily blue-emitting LEDs (typically GaN or InGaN with emissionaround 430 to 470 nm) combined with a red-emitting phosphor. A suitablered-emitting phosphor is a Eu-doped silicon nitride material, likeM₂Si₅N₈ (M=Ca, Sr, Ba), see for example WO 01/40403. Also applicationfor colored light sources is possible.

BRIEF DESCRIPTION OF THE DRAWINGS

In the text which follows, the invention is explained in more detailwith reference to a plurality of exemplary embodiments. In the drawings:

FIG. 1 shows a semiconductor component (LED) which serves as lightsource for white light, with casting resin (FIG. 1 a) and withoutcasting resin (FIG. 1 b);

FIG. 2 shows an illumination device with phosphors in accordance withthe present invention;

FIG. 3 shows the emission spectrum and reflection spectrum of a phosphorin accordance with the present invention;

FIGS. 4 to 8 show the emission spectra and reflection spectra of furtherphosphors in accordance with the present invention;

FIG. 9 shows the relationship between the volume of a unit cell and thevalue of x.

BEST MODE FOR CARRYING OUT THE INVENTION

By way of example, a structure similar to that used in WO 01/40403 isdescribed for use in a white LED together with an InGaN chip. Thestructure of such a light source for white light is specifically shownin FIG. 1 a. The light source is based on a semiconductor component(chip 1) of type InGaN with a peak emission wavelength of 400 nm, havinga first and a second electrical connection 2,3, which is embedded in anopaque base housing 8 in the region of a recess 9. One of theconnections 3 is connected to the chip 1 via a bonding wire 4. Therecess has a wall 7 which serves as reflector for the blue primaryradiation of the chip 1. The recess 9 is filled with a potting compound5 which contains a silicone casting resin (or epoxy casting resin) asits main constituents (pref. more than 80 by weight) and furthercomprises phosphor pigments 6 (pref. less than 15% by weight). There arealso further small amounts of, inter alia, methyl ether and Aerosil. Thephosphor pigments are a mixture of three pigments which emit blue, greenand red light with the green phosphor being in accordance with theinvention.

FIG. 1 b shows an embodiment of a light source with a semiconductorcomponent 10 in which the conversion into white light is effected bymeans of phosphor conversion layers 16 which are applied directly to theindividual chip. On top of a substrate 11 there are a contact layer 12,a mirror 13, a LED chip 14, a filter 15 and a phosphor layer 16, whichis excited by the primary radiation of the LED, and converts it intovisible long-wave radiation. This structural unit is surrounded by aplastic lens 17. Only the upper contact 18 of the two ohmic contacts isillustrated. Primary UV radiation of the LED is around 400 nm andsecondary radiation is emitted by a first phosphor in accordance withthe invention M_((1-c))Al2_(x)Si_(x)O_(4-x)N_(x):D_(c) with D being Euor Eu,Mn. Especially preferred is using BaAl_(2-x)Si_(x)O_(4-x)N_(x):(10%) Eu²⁺, emitting around 525 nm (or in the language of the a.m.general formula c=0,1 and D=Eu), and by a second phosphor using aNitridosilicate emitting orange-red.

FIG. 2 shows an illumination device 20. It comprises a common support21, to which a cubical outer housing 22 is adhesively bonded. Its upperside is provided with a common cover 23. The cubical housing has cutoutsin which individual semiconductor components 24 are accommodated. Theyare blue emitting light-emitting diodes with a peak emission of around450 to 470 nm. The conversion into white light takes place by means ofconversion layers 25 which are arranged on all the surfaces which areaccessible to the blue radiation. These include the inner surfaces ofthe side walls of the housing, of the cover and of the support. Theconversion layers 25 consist of phosphors which emit in the red spectralregion, and in the green spectral region using a phosphor according tothe invention and mixing up together with the non-absorbed part of theprimary radiation blue primary into white light.

Eu₂O₃ (with purity 99.99%), BaCO₃ (with purity >99.0%), SrCO₃ (withpurity >99.0%), CaCO₃ (with purity >99.0%), Al₂O₃ (with purity 99,9%),SiO₂ and Si₃N₄ were used as commercially available starting materialsfor the production of the new inventive phosphors. The raw materialswere homogeneously wet-mixed in the appropriate amounts by a planetaryball mill for 4-5 hours in isopropanol. After mixing the mixture wasdried in a stove and ground in an agate mortar. Subsequently, thepowders were fired in molybdenum crucibles at 1100-1400° C. under areducing nitrogen/hydrogen atmosphere in a horizontal tube furnace.After firing, the materials were characterized by powder X-raydiffraction (copper K-alpha line).

All samples show efficient luminescence under UV-blue excitation withemission maxima in the blue (in case of M=Ca), especially 435 to 445 nm,or green (for M=Sr or Ba), especially in case of Ba 495 to 530 nm and incase of Sr 515 to 575 nm. Two typical examples of emission andexcitation spectra can be seen in FIG. 3 and FIG. 4. FIG. 3 showsemission spectra and excitation spectra ofSrAl_(2-x)Si_(x)O_(4-x)N_(x):Eu with varying value of x. FIG. 4 showsemission/excitation spectra of BaAl_(2-x)Si_(x)O_(4-x)N_(x):Eu withvarying value of x.

By varying the amount of nitrogen, the emission can be shifted in therange 495-575 nm. Esp. Sr is sensitive to shifting (FIG. 3), while thetop of the excitation band can be shifted from below 400 nm up till 430to 465 nm, preferably to 440 nm, see M=Ba (FIG. 4). The observed shiftto higher wavelengths is the result of a center of gravity of the Eu 5 dband at lower energy and a stronger ligand-field splitting of the Eu 5 dband.

Additional fine tuning can be achieved by incorporation of Zn as anaddition to cation M, preferably not more than 30%, and at least partialreplacement of Al by Ga, preferably not more than 25%, and/or Si by Ge,preferably not more than 25%. TABLE 1 Raw materials Grade MCO₃ (M = Ca,Sr, Ba) 99.0% SiO₂ Aerosil OX50 γ-Al₂O₃ >99.995 Si₃N₄ β content: 23.3%,O˜0.7% Eu₂O₃ 99.99%

In the following the synthesis procedures for MAl2-xSixO4-xNx:(10%) Eu2+(M=Ca, Sr, Ba) are given. Possible starting materials are shown in table1.

All the oxynitride phosphors including tridymite are synthesizedaccording to the following reaction equation (gas phases neglected):(1-y)MCO₃+(2-x)/2Al₂O₃+x/4 Si₃N₄+x/4 SiO₂+y/2 Eu₂O₃→M_(1-y)Eu_(y)Al_(2-x)Si_(x)O_(4-x)N_(x)with M=Ca, Sr, Ba alone or in combination. An example is y=0,1.

For example, the compositions of Ba₉Eu_(0.1)Al_(2-x)Si_(x)O_(4-x)N_(x)are shown in the following table 2. TABLE 2 (Unit: gram) x value BaCO₃Al₂O₃ Si₃N₄ SiO₂ Eu₂O₃ 0.3 3.497 1.689 0.205 0.088 0.343 0.5 3.500 1.4920.342 0.146 0.343

The detailed experimental results showed that the luminescenceproperties of the phosphors were almost independent of exact SiO₂content. However, reducing SiO₂ content in an appropriate amount (i.e.,it can be reduced up to 1/60 of the calculated amount SiO₂) stronglyincreases phase purity as well as efficiency.

The powder mixture is fired for several hours in Mo crucibles at1100-1400° C. in a reducing atmosphere of N₂ with a small amount of H₂(10%) in the horizontal tube furnaces.

Further excitation and emission spectra of samples elucidating the roleof SiO₂ reduction are shown in FIGS. 5 to 8.

Embodiment according to FIG. 5:Ba_(0.9)Eu_(0.1)Al_(2-x)Si_(x)O_(4-x)N_(x) with x=0.3 and no SiO₂reduction. The phosphor was excited with λ_(exc)=440 nm for the emissionspectrum and monitored at λ_(mon)=530 nm for the excitation spectrum.Its main phase has BaAl₂O₄ structure.

Embodiment according to FIG. 6:Ba_(0.9)Eu_(0.1)Al_(2-x)Si_(x)O_(4-x)N_(x) with x=0.3, SiO₂ amountreduced to one-fourth of calculated SiO₂ amount (λ_(exc)=440 nm,λ_(mon)=530 nm). Its main phase has BaAl₂O₄ structure.

Embodiment according to FIG. 7:Ba_(0.9)Eu_(0.1)Al_(2-x)Si_(x)O_(4-x)N_(x) with x=0.5 and SiO₂ amountreduced to 1/16 of calculated SiO₂ amount (λ_(exc)=440 nm, λ_(mon)=530nm). Its main phase has BaAl₂O₄ structure.

Si₃N₄ was used as the source of (SiN)⁺ according to the followingreaction [1]:MCO₃+(2-x)/2 Al₂O₃+x/4 Si₃N₄+x/4SiO₂→MAl_(2-x)Si_(x)O_(4-x)N_(x){+CO₂↑}(M=Ca, Sr, Ba).

With the atomic radius decreasing from Ba to Ca it was found that thereplacement of (AlO)⁺ by (SiN)⁺ by this reaction became more difficult.Lattice parameters results show that the maximum solubility of N inBaAl₂O₄ with tridymite structure was about x≈0.6. FIG. 9 shows therelationship between unit cell volume and x values of BaAl2-xSixO4-xNxwith tridymite structure. As expected, from the smaller Si—N distance ascompared to the Al—O distance, the unit cell volume decreases withincreasing x as shown in FIG. 9. For x values remarkably larger than0.6, the unit cell volume remains almost constant, and secondary phasesare observed.

So, good results can be achieved with Ba aluminate and with x up toabout 0,6 without the need to adapt the SiO₂ amount.

Also good results are seen with Sr aluminate. Reduction (expressed asamount y) of the SiO₂ content results in an non-stoichiometric aluminateof the type MAl_(2-x)Si_(x-y)O_(4-x-2y)N_(x):Eu, preferably withy≦0,25x. Best performance is achieved with SiO₂ correction, and with xup to 0,5. The same holds true for Ca aluminate, however smaller xvalues are preferred below 0,05.

The luminescence properties of MAl_(2-x)Si_(x)O_(4-x)N_(x):10% Eu²⁺ arenow discussed in more detail. FIG. 8 shows the excitation and emissionspectra of MAl_(2-x)Si_(x)O_(4-x)N_(x):Eu²⁺ (M=Sr, Ca). The SiO₂ contentis not reduced. In more detail, FIG. 8 a shows excitation and emissionspectra of SrAl_(2-x)Si_(x)O_(4-x)N_(x):Eu²⁺ (10%) and FIG. 8 bexcitation and emission spectra of CaAl_(2-x)Si_(x)O_(4-x)N_(x):Eu₂₊(10%), each with various x.

Corresponding to the results of the relationship between unit cellvolume and x values of MAl_(2-x)Si_(x)O_(4-x)N_(x), the emission bandsshift more or less to the longer wavelength depending on the cation. Incase of Ba the shift is typically from 495 to 530 nm, in case of Sr theshift is typically from 515 to 575 nm and in case of Ca the shift istypically from 440 to 445 nm. For BaAl_(2-x)Si_(x)O_(4-x)N_(x): (10%)Eu²⁺, the emission band shifts from about 497 to 527 nm with increasingcontent x of incorporated nitrogen. The position of the excitation bandshifts accordingly from 385 to 425 nm.

For MAl_(2-x)Si_(x)O_(4-x)N_(x): (10%) Eu²⁺, with M is at least one ofSr or Ca, the amount of (SiN)⁺ does not essentially increase theemission and excitation wavelengths because almost no shift in thelattice parameters is observed. The maximum shift of emission bands ofEu is less than 10 nm corresponding to the fact of small nitrogenincorporation in case of M=Ca. Therefore, a small amount of (SiN)⁺incorporation can only exert weak influence to the local coordination ofthe Eu²⁺ ion. This discussion is understood with the addition of thenormal stoichiometric amount of SiO₂.

However, it turned out very surprisingly that the use ofunderstoichiometric amount of SiO₂ is advantageous in several cases. Theeffect of an reduced amount of SiO₂ in the reaction (1) to theincorporation of N and to the luminescence properties is most pronouncedin the case of SrAl_(2-x)Si_(x)O_(4-x)N_(x):Eu²⁺ (10%). The reason isnot yet fully understood. When the amount of SiO₂ which should be usedin reaction (1) is taken about a factor 60 lower, it is found that anincreased amount of (SiN)⁺ is incorporated into SrAl₂O₄ lattice withstuffed tridymite structure. The maximum solubility is x ≈0.5 (Table 1).As a result of N incorporation the Eu emission bands shift to longerwavelengths, up to 575 nm (FIG. 3). An evident excitation shoulder at430 nm appears at the maximum solubility of nitrogen (x=0,3 . . . 0,5),as shown in FIG. 3.

Instead of a structure derived from tridymite, also a structure derivedfrom the structure of SrSiAl₂O₃N₂ is possible. This compound wasdiscovered by Schnick in 1998. It is isotopic with silicate nitrides ofthe type LnSi₃N₅ (with Ln=La, Ce, Pr, Nd).

1. Luminescent material, preferably a phosphor for LED-applications,which is excitable in the UV-blue region from 360 to 470 nm,characterized by a Eu-doped host lattice with general compositionMaI_(2-x)Si_(x)O_(4-x)N_(x), wherein M is at least one of an alkalineearth metal chosen from the group Ca, Sr, Ba, with x≧0,002 and x≦1,5,pref. x below 0,7, and with a proportion of Eu from 0,1 to 25% of M. 2.UV-blue excitable luminescent material according to claim 1, wherein thehost lattice is derived from a tridymite structure.
 3. UV-blue excitableluminescent material according to claim 1, wherein M is Strontium and/orBarium in order to achieve green emission.
 4. UV-blue excitableluminescent material according to claim 1, wherein M is Calcium in orderto achieve blue emission.
 5. UV-blue excitable luminescent materialaccording to claim 1, wherein M is a mixture of at least two of thesemetals.
 6. UV-blue excitable luminescent material according to claim 1,wherein M comprises in addition Zn, preferably up to 40 mol-%. 7.UV-blue excitable luminescent material according to claim 1, wherein Alis replaced fully or partially by Ga.
 8. UV-blue excitable luminescentmaterial according to claim 1, wherein Si is replaced fully or partiallyby Ge, preferably not more than 25%.
 9. UV-blue excitable luminescentmaterial according to claim 1, wherein the host material is furtherdoped with Mn, the amount of co-doped Mn being preferably at most up to50% of the Eu doping.
 10. Light source (20) with a UV-blue excitableluminescent material according to claim
 1. 11. Light source with aUV-blue excitable luminescent material according to claim 1, wherein theprimary emitted light is blue and the UV-blue excitable luminescentmaterial is combined with other phosphors, in order to convert part ofthe primary emitted light into secondary emitted light of longerwavelength resulting in emitting white light.
 12. Light source with aUV-blue excitable luminescent material according to claim 1, wherein theprimary emitted radiation is UV and the UV-blue excitable luminescentmaterial is combined with other phosphors in order to convert theprimary emitted radiation into secondary emitted light of longerwavelength resulting in emitting white light.
 13. Light source accordingto claim 10, wherein the light source is an illuminating device with atleast one LED.
 14. Light source according to claim 11, wherein the otherphosphor is a red emitting phosphor.
 15. Light source according to claim12, wherein the other phosphor is a red emitting phosphor.